The invention relates generally to sound absorbent materials, and specifically to sound absorbent materials for industrial applications, such as linings on or inside enclosures around machinery.
Recently, increased attention has been focused on noise pollution, including that generated by the use of machinery, such as drills, lathes, and the like. Noise is a problem to the operator of the machine, as well as to those in the area where the machine is being operated. As a result efforts have and are being made to prevent or to reduce the noise level of machinery in order to provide a safer, quieter work area.
Attempts have been made to silence or to reduce machine noise by lining the inside of the machine, or housing surrounding the machine, with a material which will soak up or reduce the noise. It is important that the sound absorbent material be relatively inexpensive, in addition to being an efficient and effective sound absorber. If the material is too expensive, the expense will make its use prohibitive, even though it is efficient and effective, especially considering the number of machines which would use the material. Thus, an inexpensive material would find greater use, even where it was inefficient or would eventually become ineffective and have to be replaced, because of its low cost.
A typical lining material for machinery is open-cell polyurethane foam, which is a resistive sound absorber. It is a relatively inexpensive material, and it will reduce the noise level by providing a resistance to the passage of the sound emanating from the machine. But, open-cell polyurethane foam has a tendency to soak up oil and the like used to lubricate the machine. Once the cells of the sound-absorbent foam material fills with oil, the material becomes noise reflective, and so is inefficient and ineffective. Further, the accumulated oil represents a fire hazard. Another sound absorbent material is the non-woven fiberglass pad which is similar to open-cell foam in its operation, and likewise becomes ineffective and/or hazardous due to oil absorption.
Laminated acoustical material having a liquid resistant facing sheet is also known in the art. As an example, reference is made to McCluer U.S. Pat. No. 3,322,233 wherein a plastic film (1) is adhesively bonded (2) to a loosely woven fabric (3) which overlies a loose fibrous substance (4). As a backing there is a septum sheet including woven fabric (5) and an elastomeric substance (6) into which pellets (7) are embedded. However, in this type of arrangement, the loose fibrous substance (4) is required as the sound absorbent and the build-up of grime on the plastic film (1) will reduce the effectiveness of the laminate.
In a similar manner, in jet engine exhaust facilities it has become common practice to use a facing sheet over a honeycomb-type sound absorbing media. The facing sheet permits entry of sound woven therethrough for absorption by the absorbing media as the sound waves are trapped between a sound impervious backing and the facing sheet.
Examples of this type structure are found in U.S. Pat. Nos. 3,770,560 to Elder (perforated metal or plastic facing sheet); 3,374,234 to Wirt (perforated facing sheet); 3,700,067 to Dobbs (three dimensional acoustic face sheet 12); 3,630,312 to Woodward (four layer expanded metal mesh facing); 3,502,171 to Cowan (facing comprises at least two superposed and laminated woven cloth plies); 3,166,149 to Hulse (facing comprises an open-weave fiberglass screen bonded to a porous cloth or fabric layer), and 3,103,987 to Gildard (a laminate of glass fiber cloth sandwiched between layers of wire screen and faced with a perforated sheet).
My copendng application Ser. No. 539,854 (now U.S. Pat. No. 3,977,492) also discloses the concept of using facing sheet over a dead air space. There, however, the facin is a composite used for acoustical flow resistance as well as for its ability to drain away solvents, oil and water.
Unlike the situation where a fast moving gas stream is to be encountered and a honeycomb required, the laminate of my copending application is more adaptable in its usage. The acoustical material claimed in that application is assembly of a composite comprising at least one layer of woven fiberglass cloth adhesively bonded to an open mesh wire screen and placed over a non-woven fibrous mat.
Still, there are situations when it is desirable to use a laminated composite alone, without a non-woven fibrous mat. In such instances, the composite, because of its acoustical resistance, may be used over any dead air space or may even be used alone in architectural applications. However, when so used an improved composite of increased flexibility is desirable.
Therefore, the need exists for an improved sound absorbing composite which may be used over a dead air space for industrial applications such as lining on the inside of machines or the enclosures around the machines as well as for architectural applications.